It is conventional to use sheet metal ducts of rectangular cross-section for conducting pressure air in ovens, blowers and similar apparatus and for conducting suction air from exhaust hoods and the like. Such ducting can be fabricated quickly, easily and at low cost. Heretofore, however, there has not been a completely satisfactory flexible connector for such ducting whereby pressure air, suction air, hot gas or the like can be conducted between a stationary apparatus element such as a blower inlet or outlet and an apparatus element such as a nozzle or a suction inlet that undergoes a substantial amount of movement in a pair of opposite directions.
U.S. Pat. No. 3,739,491, to Creapo et al, typifies apparatus wherein a flexible duct connector of the type here under consideration would be useful but wherein an altogether different type of connection was employed to avoid the disadvantages of prior flexible connectors. That patent relates to a web dryer comprising an oven through which a straight stretch of freshly imprinted lengthwise moving web extends and wherein the web is contactlessly supported by two sets of elongated air nozzles of the type known as air bars. One set of air bars is mounted beneath the path of the web to blow upwardly against it and levitate it; the other set is mounted above the path of the web to blow downwardly against it. The air issuing from both sets of nozzles may be heated, for drying the ink on the web. When the apparatus is in operation, there is a relatively narrow space between the two sets of air bars, and therefore, to facilitate threading a web through that space, the lower set of air bars is mounted for movement between a raised operative position and a lowered web threading position. Pressure air is fed into both sets of air bars from a system of stationary ducts connected with a blower. To accommodate the up and down movement of the lower set of air bars, the stationary duct system and the lower air bar assembly have rectangular-section duct elements which project horizontally towards one another and meet at a joint defined by mating duct element surfaces that are inclined to the vertical. Because of the inclination of the plane of separation of the joint surfaces, the joint forms a closed connection when the lower air bar assembly is in its raised operative position but is open whenever the lower air bars are below that position. Although inexpensive and satisfactory for some applications, such a joint tends to leak after a period of use and is therefore not suitable for apparatus wherein leakage is not acceptable, as is usually the case with large installations.
There are also installations in which a leak-proof flexible connection is mandatory because air or other gas must be conducted to or from a movable apparatus element in all positions of its motion. Typically, it is often required that suction air be drawn from a retractable dryer hood when it is in a retracted or raised position as well as when it is in a normal operating position. Or, as exemplified by U.S. Pat. No. 3,183,607, it may be necessary to draw suction air from a movable apparatus element in one of its positions and supply it with pressure air in another position, and to apply suction and pressure alternately to a stationary duct element to which the movable apparatus element has a flexible connection.
Heretofore, flexible connectors suitable for installations of the above described character have usually been made up of flexible materials, most often coated woven materials or elastomers. Such materials are expensive, and fabricating them into ducting connectors requires techniques quite different from those needed for manufacturing and assembling sheet metal ducting itself. Connectors made of flexible materials also have serious disadvantages in service, in that they are subject to wear and ripping, and the connector must be replaced if it develops any small hole or tear, often at the cost of considerable down time in addition to the cost of the replacement connector and its installation. An additional disadvantage is that the flexible materials commonly used for such connectors have temperature limitations which make them unsuitable for oven installations and the like wherein they are contacted by very hot gases. Where high temperatures were involved, flexible connectors have been made of asbestos, but that material has become so expensive--owing to environmental protection measures needed in its manufacture--that the cost of an asbestos flexible connector is nearly prohibitive.
U.S. Pat. No. 4,130,463, to S. N. Klavir, discloses a rectangular-section duct connector that is intended for the rather specialized purpose of conducting gas between two coupled railway cars. It comprises two rectangular cross-section duct components, one of which is sector-shaped in plan view and is partially received in the other. The cost of the connector is disproportionately high for general use in rectangular-section sheet metal ducting because its two duct components are connected by a hinge joint which extends vertically along the narrow side of the sector-shaped component and which must be accurately welded to both components. Furthermore, the structure provides a clearance between adjacent matingly curved side wall surfaces of the two duct components, to enable them to pass each other as the one pivots relative to the other. That clearance, according to the patent, is intended to "minimize the escape of emissions between the surfaces," but it obviously does not prevent leakage that would be very undesirable in most ducting installations.
U.S. Pat. No. 2,805,401, to J. C. Crowley, discloses a hinge joint connection for a form of square ducting employed as a wave guide for microwave radiation. As might be expected from its very specialized application, the structure is too complicated and expensive to be suitable for sheet metal ducting.
One type of flexible connector heretofore devised and found satisfactory for a very limited range of applications has comprised a pair of rectangular cross-section duct elements that had a telescoped connection with one another. The outside cross-section dimensions of the inner telescoped duct element were sufficiently smaller than the inside dimensions of the outer one to permit limited relative swinging motion between the telescoped duct elements. To provide a seal between them, a resiliently compressible gasket was secured to the outer duct element, in surrounding relation to the inner one. Because of its resilient compressibility, the gasket maintained a seal while accommodating swinging of the telescoped duct elements, but the angle through which they could be swung was obviously rather small, and therefore this prior flexible connector was not suitable for a compact installation that could accommodate a substantial travel of the movable apparatus element with which it was connected.